Method of generating a multiscale contrast enhanced image

ABSTRACT

At least one approximation image is created of the image at one or multiple scales. Translation difference images are created by pixel-wise subtracting the values of an approximation image at scale s and the values of a translated version of the approximation image. A non-linear modification is applied to the values of the translation difference image (s) and at least one enhanced center difference image at a specific scale is computed by combining the modified translation difference images at that scale or a smaller scale with weights w i,,j . An enhanced image is computed by applying a reconstruction algorithm to the enhanced center difference images. The non-linear modification of the values of the translation difference images is steered by (a) characteristic (s) computed out of the approximation image (s) at least one scale.

FIELD OF THE INVENTION

The present invention relates to a method for enhancing the imagequality of an image that is represented by a digital signal.

BACKGROUND OF THE INVENTION

Commonly images represented by a digital signal such as medical imagesare subjected to image processing during or prior to displaying or hardcopy recording.

The conversion of grey value pixels into values suitable forreproduction or displaying may comprise a multi-scale image processingmethod (also called multi-resolution image processing method) by meansof which the contrast of the image is enhanced.

According to such a multi-scale image processing method an image,represented by an array of pixel values, is processed by applying thefollowing steps. First the original image is decomposed into a sequenceof detail images at multiple scales and occasionally a residual image.Next, the pixel values of the detail images are modified by applying tothese pixel values at least one conversion. Finally, a processed imageis computed by applying a reconstruction algorithm to the residual imageand the modified detail images.

There are limits for the behavior of the conversion functions. Greyvalue transitions in the image can be distorted to an extent that theappearance becomes unnatural if the conversion functions are excessivelynon-linear. The distortions are more pronounced in the vicinity ofsignificant grey level transitions, which may result in overshoots atstep edges and loss of homogeneity in regions of low variance facingstrong step edges. The risk of creating artifacts becomes moresignificant for CT images since they have sharper grey leveltransitions, e.g. at the interface of soft tissue and contrast media.One has to be careful using the multi-scale techniques on CT images.

A multi-scale contrast enhancement algorithm which results in a contrastenhanced image while preserving the shape of the edge transitions hasbeen described in co-pending European patent application 06 125 766.3filed Dec. 11, 2006.

In one embodiment of this method translation difference images of atleast one approximation image of the image are created at one ormultiple scales. Next, translation difference images are non linearlymodified. Then at least one enhanced center difference is image at aspecific scale is computed by combining modified translation differenceimages at that scale or at a smaller scale. Finally an enhanced image iscomputed by applying a reconstruction algorithm to the enhanced centerdifference images.

It is an object of the present invention to further enhance this method.

SUMMARY OF THE INVENTION

The above-mentioned further enhancement is obtained by a method havingthe specific steps set out in claim 1. Specific elements for preferredembodiments of the invention are set out in the dependent claims.

In the context of the present invention specific terms are defined asfollows:

Multi-scale Decomposition Mechanism:

A multi-scale (or multi-resolution) decomposition of an image is aprocess that computes detail images of said image at multiple scales ofa grey value image. A multi-scale decomposition mechanism generallyinvolves filter banks for computing the detail images. Well-knowntechniques are for example: the Laplacian pyramid, the Burt pyramid, theLaplacian stack, the wavelet decomposition, QMF filter banks.

Approximation Image:

An approximation image is a grey value image that represents theoriginal grey value image at the same or a larger scale, or at the sameor a lower resolution. An approximation image at a specific scale isequivalent to the original grey value image in which all details at thatscale have been omitted (Mallat S. G., “A Theory for MultiresolutionSignal Decomposition: The Wavelet Representation”, IEEE Trans. OnPattern Analysis and Machine Intelligence, vol. 11, no. 7, Jul. 1989).

Detail Image:

A detail image is defined as the difference of information between anapproximation image at a certain scale and an approximation image at asmaller scale.

Conversion Operator:

A conversion operator is an operator which generates the pixel-wisemodification of the detail pixel values as an intermediate step tocreate a contrast enhanced version of the grey value image. Such anoperator has for example been described in European patent EP 527 525.The modification is defined by a conversion function and can e.g. beimplemented as a look up table or as a multiplicative amplification.

Translation Difference Image:

The translation difference images at a scale s are a measurement ofelementary contrast in each pixel of an approximation image at scale s.They can be computed by taking the difference of the approximation imageat that scale s and a translated version. Other computations forelementary contrast are possible, e.g. the ratio of pixel with aneighboring pixel can be used in case the processing steps are precededby an exponential transform and followed by a log transform.

Center Difference Image:

A center difference image is computed by applying a combining operator(for example the summation) to translation difference images.

The combining operator can be a linear or non-linear function ofcorresponding pixel values in the translation difference images.

The prior art method is enhanced by steering the non-linear modificationof the values of the translation difference images by one or morecharacteristics that are computed out of the approximation images atleast one scale.

Examples of these characteristics that are described further on are theaverage grey value in an area, the local standard deviation etc.

Another example is the presence of predefined image structures orabnormal patterns in an approximation image. An example of such apattern is a microcalcification in a mammographic image.

The detection of these abnormal patterns can be performed by severaltechniques reaching from simple filtering to a complex computer aideddetection algorithm. For example by means of a binary localization maskthe presence of these abnormal patterns can be used to steer thenon-linear modification.

In one embodiment the approximation image mentioned in claim 1 isfiltered before translation difference image are combined out of it.Filtering can be based on the characteristics of the approximationimages and/or a detail image at the same or a coarser scale.

Gain adjustment can be obtained by filtering enhanced center differenceimages. Filtering may be based on the characteristics of anapproximation image or a detail image.

Furthermore the enhanced center difference images can be subjected to apixel-wise transformation such as a normalization of amplitudes of theenhanced multi-scale representation.

Different embodiments are described in which translation differenceimages are computed out of approximation images at the same scale, outof the original image or out of a combination of approximation images atdifferent scales.

The present invention is generally implemented as a computer programproduct adapted to carry out the method of any of the claims when run ona computer and is stored on a computer readable medium.

The methods of the present invention can be applied for enhancing theimage quality of medical images such as mammographic images, imagesobtained by computed tomography etc.

Further advantages and implementations of the embodiments of the presentinvention will be explained in the following description and will beillustrated by the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a multi-resolution image processing scheme with two controlpaths.

FIG. 2 shows an advanced enhancement functional block,

FIGS. 3 and 5 illustrate different implementations of themulti-resolution image processing method according to the presentinvention,

FIG. 4 illustrates the image enhancement step of the multi-resolutionimage processing method illustrated in FIG. 3,

FIG. 6 illustrates the image enhancement step of the multi-resolutionimage processing method illustrated in FIG. 5,

FIG. 7 is a legend pertaining to the symbols used in the above figures.

DETAILED DESCRIPTION OF THE INVENTION

The contrast enhancement algorithm of the present invention isapplicable to all multi-scale detail representation methods from whichthe original image can be computed by applying the inversetransformation.

It is applicable to the reversible multi-scale detail representationthat can be computed as a weighted sum of translation difference images.

The weighing factors and the translation offsets of the translationdifference images can be deducted from the multi-scale is decompositionin such a way that the resulting weighted sum of the translationdifference images is identical to the detail pixel values.

For these multi-scale detail representations the contrast can beenhanced by applying the conversion operator to the translationdifferences before the weighted sum is computed.

To compute the weighted sum of translation difference images, theapproximation image at the same scale (or resolution level) or theapproximation images at the smaller scales (or finer resolution levels)can be used.

State-of-the-art multi-scale contrast enhancement algorithms decomposean image into a multi-scale representation comprising detail imagesrepresenting detail at multiple scales and a residual image.

Some of the important multi-scale decompositions are the waveletdecomposition, the Laplacian-of-Gaussians (or LoG decomposition), theDifference-of-Gaussians (or DoG) decomposition and the Burt pyramid.

The wavelet decomposition is computed by applying a cascade of high-passand low-pass filters followed by a subsampling step.

The high-pass filter extracts the detail information out of anapproximation image at a specific scale.

In the Burt pyramid decomposition the detail information is extractedout of an approximation image at scale k by subtracting the upsampledversion of the approximation image at scale k+1.

In a state of the art methods as the one disclosed in EP 527 525 acontrast enhanced version of the image is created by conversion of thepixel values in the detail images followed by multi-scalereconstruction.

All above implementations of multiscale decomposition have a commonproperty. Each pixel value in the detail images can be computed out ofan approximation image by combining the pixel values in a movingneighborhood.

In the above cases the combining function is a weighted sum.

For the wavelet decomposition the pixel values in the detail image atscale k are computed as:d _(k+1)=⇓(h _(d) *g _(k))g _(k+1)=⇓(l _(d) *g _(k))

with h_(d) a high-pass filter, l_(d) a low-pass filter, * theconvolution operator and ⇓ the subsampling operator (i.e. leaving outevery second row and column).

For the wavelet reconstruction the enhanced approximation image at scalek is computed as:h _(k) =l _(r)*(⇑h _(k+1))+h _(r)*(⇑f(d _(k+1)))

with h_(r) a high-pass filter, l_(r) a low-pass filter and ⇑ theupsampling operator (i.e. inserting pixels with value 0 in between anytwo rows and columns).

For the Burt decomposition the pixel values in the detail image at scalek are computed as:d _(k) =g _(k)−4g*(⇑g _(k+1))ord _(k) =g _(k)−4g*(⇑(⇓(g*g _(k))))ord _(k)=(1−4g*(⇑(⇓g)))*g _(k)

with g a Gaussian low-pass filter and 1 the identity operator.

For the Burt reconstruction the enhanced approximation image at scale kis computed as:h _(k)=4g*(⇑(⇑h _(k+1))+f(d _(k))with f(x) the conversion operator.

The Multi-scale Detail Pixel Values as Weighted Sums

Suppose that in the Burt multi-scale decomposition a 5×5 Gaussian filteris used with coefficients w_(k,1) with k=−2, . . . 2 and 1=−2, . . . ,2, the subsampling operator removes every second row and column and theupsampling operator inserts pixels with value 0 in between any two rowsand columns.

The pixel at position i,j in the approximation image g_(k+1) is computedas:

${g_{k + 1}\left( {i,j} \right)} = {\sum\limits_{s = {- 2}}^{2}{\sum\limits_{t = {- 2}}^{2}{w_{s,t}{g_{k}\left( {{{2i} + s},{{2j} + t}} \right)}}}}$

The pixel at position i,j in the upsampled image u_(k) is computed as:

${u_{k}\left( {i,j} \right)} = \left\{ \begin{matrix}{\sum\limits_{s = {- 2}}^{2}{\sum\limits_{t = {- 2}}^{2}{w_{s,t}{g_{k}\left( {{i + s},{j + t}} \right)}}}} & {{if}\mspace{14mu} i\mspace{14mu}{and}\mspace{14mu} j\mspace{14mu}{are}\mspace{14mu}{even}} \\0 & {otherwise}\end{matrix} \right.$The pixel at position i,j in the upsampled, smoothed image gu_(k) iscomputed as:

${{gu}_{k}\left( {i, j} \right)} = \left\{ \begin{matrix}{\sum\limits_{m = {\{{{- 2},0,2}\}}}{\sum\limits_{n = {\{{{- 2},0,2}\}}}{w_{m,n}{\sum\limits_{s = {- 2}}^{2}{\sum\limits_{t = {- 2}}^{2}{w_{s,t}{g_{k}\begin{pmatrix}{{i + s + m},} \\{j + t + n}\end{pmatrix}}\begin{matrix}{{if}\mspace{14mu} i\mspace{14mu}{and}} \\{\;{j\mspace{14mu}{are}\mspace{14mu}{even}}}\end{matrix}}}}}}} \\{{\sum\limits_{m = {\{{{- 1},1}\}}}{\sum\limits_{n = {\{{{- 2},0,2}\}}}{w_{m,n}{\sum\limits_{s = {- 2}}^{2}{\sum\limits_{t = {- 2}}^{2}{w_{s,t}{g_{k}\begin{pmatrix}{{i + s + m},} \\{j + t + n}\end{pmatrix}}\begin{matrix}{{if}\mspace{14mu} i\mspace{14mu}{is}\mspace{14mu}{odd}} \\{{and}\mspace{14mu} j\mspace{14mu}{is}\mspace{14mu}{even}}\end{matrix}}}}}}}\mspace{14mu}} \\{\sum\limits_{m = {\{{{- 2},0,2}\}}}{\sum\limits_{n = {\{{{- 1},1}\}}}{w_{m,n}{\sum\limits_{s = {- 2}}^{2}{\sum\limits_{t = {- 2}}^{2}{w_{s,t}{g_{k}\begin{pmatrix}{{i + s + m},} \\{j + t + n}\end{pmatrix}}\begin{matrix}{{if}\mspace{14mu} i\mspace{14mu}{is}\mspace{14mu}{even}} \\{{and}\mspace{14mu} j\mspace{14mu}{is}\mspace{14mu}{odd}}\end{matrix}}}}}}} \\{\sum\limits_{m = {\{{{- 1},1}\}}}{\sum\limits_{n = {\{{{- 1},1}\}}}{w_{m,n}{\sum\limits_{s = {- 2}}^{2}{\sum\limits_{t = {- 2}}^{2}{w_{s,t}{g_{k}\begin{pmatrix}{{i + s + m},} \\{j + t + n}\end{pmatrix}}\begin{matrix}{{if}\mspace{14mu} i\mspace{14mu}{and}} \\{j\mspace{14mu}{are}\mspace{14mu}{odd}}\end{matrix}}}}}}}\end{matrix} \right.$Finally, the pixel at position i,j in the detail image d_(k) is computedas:

${d_{k}\left( {i, j} \right)} = \left\{ \begin{matrix}{{g_{k}\left( {i,j} \right)} - {4{\sum\limits_{m = {\{{{- 2},0,2}\}}}{\sum\limits_{n = {\{{{- 2},0,2}\}}}{w_{m,n}{\sum\limits_{s = {- 2}}^{2}{\sum\limits_{t = {- 2}}^{2}{w_{s,t}{g_{k}\begin{pmatrix}{{i + s + m},} \\{j + t + n}\end{pmatrix}}}}}}}}}} & \begin{matrix}{{if}\mspace{14mu} i\mspace{14mu}{and}} \\{\;{j\mspace{14mu}{are}\mspace{14mu}{even}}}\end{matrix} \\{{g_{k}\left( {i,j} \right)} - {4{\sum\limits_{m = {\{{{- 1},1}\}}}{\sum\limits_{n = {\{{{- 2},0,2}\}}}{w_{m,n}{\sum\limits_{s = {- 2}}^{2}{\sum\limits_{t = {- 2}}^{2}{w_{s,t}{g_{k}\begin{pmatrix}{{i + s + m},} \\{j + t + n}\end{pmatrix}}}}}}}}}} & \begin{matrix}{{if}\mspace{14mu} i\mspace{14mu}{is}\mspace{14mu}{odd}} \\{{and}\mspace{14mu} j\mspace{14mu}{is}\mspace{14mu}{even}}\end{matrix} \\{{g_{k}\left( {i,j} \right)} - {4{\sum\limits_{m = {\{{{- 2},0,2}\}}}{\sum\limits_{n = {\{{{- 1},1}\}}}{w_{m,n}{\sum\limits_{s = {- 2}}^{2}{\sum\limits_{t = {- 2}}^{2}{w_{s,t}{g_{k}\begin{pmatrix}{{i + s + m},} \\{j + t + n}\end{pmatrix}}}}}}}}}} & \begin{matrix}{{if}\mspace{14mu} i\mspace{14mu}{is}\mspace{14mu}{even}} \\{{and}\mspace{14mu} j\mspace{14mu}{is}\mspace{14mu}{odd}}\end{matrix} \\{{g_{k}\left( {i,j} \right)} - {4{\sum\limits_{m = {\{{{- 1},1}\}}}{\sum\limits_{n = {\{{{- 1},1}\}}}{w_{m,n}{\sum\limits_{s = {- 2}}^{2}{\sum\limits_{t = {- 2}}^{2}{w_{s,t}{g_{k}\begin{pmatrix}{{i + s + m},} \\{j + t + n}\end{pmatrix}}}}}}}}}} & \begin{matrix}{{if}\mspace{14mu} i\mspace{14mu}{and}} \\{j\mspace{14mu}{are}\mspace{14mu}{odd}}\end{matrix}\end{matrix} \right.$

Generally, the pixel at position i,j in the detail image d_(k) can becomputed as a weighted sum of pixels in the approximation image at thesame or smaller scale k, k−1, k−2, . . . :

${d_{k}\left( {i,j} \right)} = {{g_{l}\left( {{ri},{rj}} \right)} - {\sum\limits_{m}{\sum\limits_{n}{v_{m,n}{g_{l}\left( {{{ri} + m},{{rj} + n}} \right)}}}}}$

with 1ε{0, . . . , k} and r=subsampling_factor^((1−k))

Because

${\sum\limits_{m}{\sum\limits_{n}v_{m,n}}} = 1$

the pixel at position i,j in the detail image d_(k) can be computed as:

${d_{k}\left( {i,j} \right)} = {{g_{l}\left( {{ri},{rj}} \right)} - {\sum\limits_{m}{\sum\limits_{n}{v_{m,n}{g_{l}\left( {{{ri} + m},{{rj} + n}} \right)}}}}}$${d_{k}\left( {i,j} \right)} = {{\sum\limits_{m}{\sum\limits_{n}{v_{m,n}{g_{l}\left( {{ri},{rj}} \right)}}}} - {\sum\limits_{m}{\sum\limits_{n}{v_{m,n}{g_{l}\left( {{{ri} + m},{{rj} + n}} \right)}}}}}$${d_{k}\left( {i,j} \right)} = {{c_{k}\left( {i,j} \right)} = {\sum\limits_{m}{\sum\limits_{n}{v_{m,n}\left( {{g_{l}\left( {{ri},{rj}} \right)} - {g_{l}\left( {{{ri} + m},{{rj} + n}} \right)}} \right)}}}}$

The term g_(l)(ri,rj)−g_(l)(ri+m,rj+n) is called a translationdifference.

It expresses the difference in pixel value between a central pixel and aneighboring pixel in an approximation image. It is a measure of localcontrast.

The weighted sum of the translation differences is called a centredifference c_(k)(i,j).

In a similar way it can be proven that the detail images in othermulti-scale decomposition methods can also be represented as acombination of translation difference images.

The Conversion Operation

In state-of-the-art methods like the one disclosed in EP 527 525contrast enhancement is obtained by applying a conversion operator f(x)to the detail image d_(k) or, equivalently:

${f\left( {d_{k}\left( {i,j} \right)} \right)} = {f\left( {{g_{l}\left( {{ri},{rj}} \right)} - {\sum\limits_{m}{\sum\limits_{n}{v_{m,n}{g_{l}\left( {{{ri} + m},{{rj} + n}} \right)}}}}} \right)}$

An example of such a conversion operator is the sigmoid function.Another example of such conversion operator is the contrast enhancementfunction like the one disclosed in EP 527 525. The shape of theconversion operator depends on the specific requirements of theenhancement which is intended to amplify the low-value detail pixel morethan the high-value detail pixels.

The conversion step may cause deformations of the shape of the edgetransitions in the reconstructed, contrast enhanced image. The reason isthe non-linearity of the conversion function.

Generally, the following applies to non-linear functions:

f(x + y) ≠ f(x) + f(y) or${f\left( {\sum\limits_{i}x_{i}} \right)} \neq {\sum\limits_{i}{f\left( x_{i} \right)}}$

State-of-the-art algorithms first compute the pixel values in the detailimage d_(k) as weighted sums and apply the conversion step afterwards.

By rewriting the pixel values in the detail image d_(k) as a weightedsum of translation differences, it is possible to apply the conversionstep before the summation instead of afterwards.

Contrast enhancement is now obtained by applying the conversion step tothe translation differences:

${f\left( {d_{k}\left( {i,j} \right)} \right)} = {\sum\limits_{m}{\sum\limits_{n}{v_{m,n}{f\left( {{g_{l}\left( {{ri},{rj}} \right)} - {g_{l}\left( {{{ri} + m},{{rj} + n}} \right)}} \right)}}}}$

In this way the shape of the edge transitions is better preserved in thecontrast enhanced, reconstructed image.

If for every scale k the detail image at that scale is computed out ofthe full resolution image g₀, and enhancement is applied to the centerdifferences, then the shapes of the edge transitions are best preservedafter reconstruction.

Different implementations of the present invention are illustrated inFIGS. 1, 3 and 5 with the corresponding enhancement steps beingillustrated in FIGS. 2, 4 and 6.

FIG. 1 shows a multi-scale image processing scheme with the centerdifference images computed out of the approximation images at the samescale. This figure shows an extended scheme for multi-resolution imageprocessing provided with control paths.

Functional Blocks A_(k)

With the functional blocks A_(k) desired characteristics can beextracted out of the approximation images g_(k) and/or g_(k+1). Thesecharacteristics can be used as control inputs for the LUT operation inthe enhancement block E and/or for optional adaptive filtering of theapproximation images and/or the enhanced center difference images.

An approximation image can be used as input for determining any usefulcharacteristic (e.g. histogram, filter coefficients, standard deviationimage, gradient image, . . . ).

An example of such a characteristic is an averaging filter to computethe average grey value in a specific neighborhood which makes grey valuedependent enhancement possible.

Another characteristic can be the local standard deviation which willresult in image activity dependent enhancement.

Still another example is the computation of a gradient image out of anapproximation image to perform gradient-driven image enhancement for thepurpose of noise reduction.

State of the art multi-scale image processing methods can be used toreduce the noise in a digital image by filtering the detail images. Astraightforward implementation is local averaging of the detail imagesso as to reduce differences between detail coefficients that are due tonoise.

However, these straightforward methods are not satisfactory with respectto reducing the noise in the image while retaining the edges, e.g. smalldetails with low contrast can become even less visible.

Better results can be achieved by adaptive filtering (e.g. weightedaveraging) of the enhanced center difference images steered by the localgradient. The steering block A_(k) can thus be used to compute gradientimages out of approximation images.

Functional Block B

The functional block B is used to enhance the residual image g_(L). Thisblock applies an optional transform to the residual image to generatethe enhanced approximation image h_(L), at scale L.

An example of such a transform is a gain adjustment to limit the dynamicrange. The residual approximation image is used as input, the output isan enhanced residual image.

The pixel value range of an input image may be too large to display allimage regions with sufficient contrast resolution. Reducing the relativecontribution of the very low frequency components in the image withrespect to the high and medium frequency components will improve thecontrast of all relevant image features.

FIG. 2 is a detailed view of the enhancement functional block used inthe method illustrated in FIG. 1.

Functional Block LUT

This transformation block is used to create an enhanced version of thecenter difference images by modifying the translation differences d.

As control input a combination can be used of:

-   -   A_(k−m)(g_(k−m), ⇑(g_(k+1−m)))    -   B_(g)(g_(k−m))    -   C_(k)(d_(k−m), ⇑(d_(k+1−m)))

The LUT operator can be implemented as a n-dimensional look up table oras an analytic function or as an adaptive filter or as any combinationof the previous ones.

-   -   Input: (optionally filtered) translation differences.    -   Optional inputs:        -   A_(k−m)(g_(k−m), ⇑(g_(k+1−m)))        -   B_(g)(g_(k−m))        -   C_(k−m)(d_(k−m), ⇑(d_(k+1−m)))    -   Output: enhanced translation differences.    -   Application:    -   Visualization of a digital image is generally improved by        amplifying the contrast of subtle image features, and at the        same time attenuating the strong components without the risk of        omitting information.    -   This contrast equalization is done by generating enhanced        multi-scale center difference images.    -   For this enhancement, the LUT operator is used to generate        enhanced translation differences and these are combined to        create enhanced center difference images.    -   The degree of enhancement can be steered by any characteristic        derived from the approximation images or detail images.    -   By using the filtered grey value approximation image itself        B_(g)(g_(k−m)), a grey-value dependent enhancement can be        applied. For example, it can be useful to enhance the subtle        contrasts more in the brighter image regions than the ones in        the dark regions (e.g. to make the fine image structures of a        mammographic image more visible within the fibroglandular        tissue).

A first optional step is the adaptive filtering of the approximationimage g_(k−m). The adaptation components can be formed by A_(k)(g_(k−m),⇑(g_(k+1−m))).

The grey values of the approximation image g_(k) can be used directly ascontrol input for the LUT component.

Also the steering component A_(k)(g_(k−m), ⇑(g_(k+1−m))) can be used asadditional control inputs for the LUT.

Adaptive Filter K_(d)

The functional block K_(d) is an optional adaptive or non-adaptivefiltering of the enhanced center difference images.

This adaptive filter can be based on the characteristics of theapproximation images at the same scale and/or coarser scaleA_(k)(g_(k−m), ⇑(g_(k+1−m))) and/or on the characteristics of the detailimages at the same scale and/or coarser scaleC_(k)(d_(k−m),⇑(d_(k+1−m))).

An example is adaptive smoothing of the enhanced center difference imagewhere the steering input A_(k)(g_(k−m), ⇑(g_(k+1−m))) is e.g. a gradientimage of the approximation image at the coarser scale.

-   -   Input: enhanced center difference image.    -   Optional inputs:        -   characteristics of the approximation image at the scale k−m            and/or scale k+1−m            -   (with m=0                multi-scale processing scheme in FIG. 1; with m> number                of scales −1                multi-scale processing scheme in FIG. 3; for                intermediate values see FIG. 5)        -   characteristics of the detail image at the scale k−m and/or            scale k+1−m            -   (with m=0                multi-scale processing scheme in FIG. 1, with m> number                of scales −1                multi-scale processing scheme in FIG. 1).    -   Output: filtered enhanced center difference image.    -   Application: It is one of the most apparent merits of digital        image processing, that contrast can be raised to any feasible        level. However, noise is amplified to the same extent. For that        reason secondary control mechanisms are preferably introduced to        reduce the amount of contrast enhancement in areas where strong        contrast enhancement is undesirable.    -   For this purpose the enhanced center difference images can be        smoothed in a direction perpendicular to the local gradient        (derived from the approximation images A_(k)(g_(k−m),        ⇑(g_(k+1−m)))). This reduces the noise in the resulting image        while retaining and even enhancing the edges in the result        image.

Optional Transformation Block L_(d)

-   -   The functional block L_(d) applies an optional pixel-wise        transform to the enhanced center difference images.    -   Input: enhanced center difference image.    -   Output: transformed, enhanced center difference image.    -   Application:    -   In case an overall increase of the sharpness of the resulting        image is preferred, this can be achieved by amplification of the        finest, enhanced center difference images.    -   Another example of such an optional transform is the        normalization of the amplitudes of the enhanced center        difference images.

The Functional Block B_(g)

-   -   The functional block B_(g) is an optional adaptive or        non-adaptive filtering of the approximation images at each scale        before the center difference images are computed.    -   The adaptive filter can be based on the characteristics of the        approximation images at the same scale and/or coarser scale        A_(k)(g_(k), ⇑(g_(k+1))) and/or on the characteristics of the        detail images at the same scale and/or coarser scale        C_(k)(d_(k), ⇑(d_(k+1))).

Input: approximation image.

Optional inputs are

-   -   characteristics of the approximation image at the scale k−m or        scale k+1−m    -   (with m=0        multi-scale processing scheme in FIG. 1; with m> number of        scales −1        multi-scale processing scheme in FIG. 3; for intermediate values        see FIG. 5).    -   characteristics of the detail image at the scale k−m and/or        scale k+1−m    -   (with m=0        multi-scale processing scheme in FIG. 1,    -   with m>number of scales −1        multi-scale processing scheme in FIG. 1).

Output: filtered approximation image.

This embodiment can be used as an anti-aliasing measure. Aliasing is acommon problem with the conventional image enhancement processes.Aliasing is produced by the downsampling and upsampling is of theapproximation images.

If the detail images are not modified, these aliasing effects arecancelled out in the reconstruction process. However, if the adaptivefiltering and the LUT conversion are applied to the detail images, thereis no proper cancellation of the aliasing effects in the reconstructionprocess. This will generate artifacts in the enhanced digital image.

The appearance of these aliasing artifacts is reduced by computing theenhanced center difference images instead of the direct enhancement ofthe detail images.

However, even better image enhancement results can be achieved bypre-filtering the approximation images g_(k) before computing thetranslation differences.

In the embodiments shown in FIGS. 1 and 2 there are no control pathsusing the detail coefficients because the detail images as such are notcomputed. Similar characteristics can be derived from the approximationimages.

The adaptive filters K_(d) and B_(g) could also be controlled by thedetail images.

FIG. 3 shows a similar extended scheme for multi-resolution imageprocessing as FIG. 1, the difference being that all the enhancedtranslation difference images are computed out of the original, fullresolution image instead of the different approximation images.

Similar control paths are shown.

The repeated subsampling blocks v^(m) can be incorporated in theenhancement block E for reason of computation efficiency. v^(m) meansthat the input image of this block is subsampled m times.

FIG. 4 is similar to FIG. 2 with control path A_(k)(g₀).

FIG. 5 and FIG. 6 show the hybrid implementation which is a combinationof the multi-scale image processing schemes showed in FIG. 1 and FIG. 3.

The drawings include subsampling to create pyramidal multi-scaledecompositions. Subsampling may be omitted to create funnel-likedecompositions.

In the description the indices of A_(k) and C_(k) corresponds with thescale of the used approximation and detail images (scale k and k+1)except in the full resolution multi-scale image processing scheme ofFIG. 3. In this last scheme the images at scale 0 are used.

1. A method of enhancing the contrast of a detail image that isrepresented by a digital signal, the method comprising: at least oneapproximation image is created at one or multiple scales, anapproximation image at a scale representing the grey values of saiddetail image in which all details at that scale have been omitted,translation difference images are created by pixel-wise subtracting thevalues of an approximation image at a scale and the values of atranslated version of said approximation image, a non-linearmodification is applied to the values of said translation differenceimages to generate modified translation difference images, at least oneenhanced center difference image at a specific scale is computed bycombining said modified translation difference images at that scale or asmaller scale with weights, said weights being selected in accordancewith an image decomposition algorithm such that, if the enhancement bynon-linear modification is not performed, an equivalent of themulti-scale decomposition of said image would be obtained in which theat least one center difference image would be equal to the detail imageat the specific scale, an enhanced image is computed by applying areconstruction algorithm to the enhanced center difference image,wherein at least one of the following actions is performed: (1) saidnon-linear modification of the values of said translation differenceimages is steered by a characteristic computed out of said approximationimage at at least one scale, (2) said approximation image at a scale isfiltered to generate a filtered approximation image, (3) said enhancedcenter difference image is filtered.
 2. A method according to claim 1wherein said characteristic is the average grey value in a predefinedimage area.
 3. A method according to claim 1 wherein said characteristicis the local standard deviation.
 4. A method according to claim 1wherein said characteristic is the presence of predefined imagestructures or abnormal patterns in said approximation image.
 5. A methodaccording to claim 1 wherein said filtering of said approximation imagesat a scale is based on the characteristics of approximation images atsaid scale or a coarser scale.
 6. A method according to claim 1 whereinsaid filtering of said approximation images is based on thecharacteristics of detail images at said scale or a coarser scale.
 7. Amethod according to claim 1 wherein a pixel-wise transformation orfiltering is applied to the residual image being the approximation imageat the lowest scale.
 8. A method according to claim 1 wherein saidfiltering of said enhanced center difference images is based on thecharacteristics of said approximation images at the same or a coarserscale.
 9. A method according to claim 1 wherein said filtering of saidenhanced center difference images is steered by the characteristics ofsaid detail image at the same or a coarser scale.
 10. A method accordingto claim 1 wherein said filtering of said enhanced center differenceimages is adaptive smoothing obtained by steering said filtering bymeans of a gradient image of an approximation image at said scale or acoarser scale.
 11. A method according to claim 1 wherein said enhancedcenter difference images are subjected to a pixel-wise transformation.12. A method according to claim 11 wherein said transformation is anormalization of the amplitudes of said enhanced center differenceimages.
 13. A method according to claim 1 wherein a translationdifference image at a specific scale is computed out of an approximationimage at the same scale.
 14. A method according to claim 1 modified inthat all of said translation difference images are computed out of theoriginal image.
 15. A method according to claim 1 wherein a translationdifference image at a scale k is computed out of an approximation imageat scale m, wherein m represents a scale between scale 1 and scale k−1.16. A method according to claim 1 wherein the center difference imagesare identical to the multi-scale detail images.
 17. A method accordingto claim 1 wherein said image is a mammographic image.
 18. A methodaccording to claim 1 wherein said image is a computed tomography image.19. A computer program product embodied on a non-transitorycomputer-readable medium adapted to carry out a method of enhancing thecontrast of a detail image that is represented by a digital signal whenrun on a computer, the method comprising: creating at least oneapproximation image one or multiple scales, an approximation image at ascale representing the grey values of said detail image in which alldetails at that scale have been omitted, creating translation differenceimages by pixel-wise subtracting the values of an approximation image ata scale and the values of a translated version of said approximationimage, applying a non-linear modification to the values of saidtranslation difference images to generate modified translationdifference images, computing at least one enhanced center differenceimage at a specific scale by combining said modified translationdifference images at that scale or a smaller scale with weights,_(:)said weights being selected in accordance with an image decompositionalgorithm such that, if the enhancement by non-linear modification isnot performed, an equivalent of the multi-scale decomposition of saidimage would be obtained in which the at least one center differenceimage would be equal to the detail image at the specific scale, andcomputing an enhanced image by applying a reconstruction algorithm tothe enhanced center difference image, wherein at least one of thefollowing actions is performed: (1) said non-linear modification of thevalues of said translation difference images is steered by acharacteristic computed out of said approximation image at at least onescale, (2) said approximation image at a scale is filtered to generate afiltered approximation image, (3) said enhanced center difference imageis filtered.
 20. A non-transitory computer readable medium comprisingcomputer executable program code adapted to carry out a method ofenhancing the contrast of a detail image that is represented by adigital signal, the method comprising: creating at least oneapproximation image one or multiple scales, an approximation image at ascale representing the grey values of said detail image in which alldetails at that scale have been omitted, creating translation differenceimages by pixel-wise subtracting the values of an approximation image ata scale and the values of a translated version of said approximationimage, applying a non-linear modification to the values of saidtranslation difference images to generate modified translationdifference images, computing at least one enhanced center differenceimage at a specific scale by combining said modified translationdifference images at that scale or a smaller scale with weights,: saidweights being selected in accordance with an image decompositionalgorithm such that, if the enhancement by non-linear modification isnot performed, an equivalent of the multi-scale decomposition of saidimage would be obtained in which the at least one center differenceimage would be equal to the detail image at the specific scale, andcomputing an enhanced image by applying a reconstruction algorithm tothe enhanced center difference image, wherein at least one of thefollowing actions is performed: (1) said non-linear modification of thevalues of said translation difference images is steered by acharacteristic computed out of said approximation image at at least onescale, (2) said approximation image at a scale is filtered to generate afiltered approximation image, (3) said enhanced center difference imageis filtered.